Resistance element



B. A. WARM EY RESISTANCE ELEMENT Jam 9, 1951 Original Filed March 12, 1945 INVENTORQ BERNARD A. WARMEY BY W M M ,ATTY.

Patented Jan. 9, 1951 RESISTANCE ELEMENT Bernard A. Warmey, Delavan, Wis., assignor to The George W. Borg Corporation, Delavan, Wis.,

a corporation of Delaware Original application March 12, 1945, Serial No. 582,319. Divided and this application May 4,

v 1948, Serial No. 24,950 a I e The present invention relates in general to r sistance elements, but more in particular to resistance elements which have the form of a compound helix, and the object of the invention is a new and improved resistance element of this character and a new and improved methodand apparatus for manufacturing the same. This application is a division of application Serial No. 582,319, filed March 12, 1945.

A resistance element as referred to hereinm-ay be made by winding a resistance wire on a straight core, forming what may be referred to as a'ininor helix, and by then winding the core on a mandrel to form the major helix. The resistance elment thus formed, after removal from the mandrel, may be used in the construction of a rheostat or potentiometer by mounting it in a suitable casing equipped with a rotatable slider or contact member which is adapted to travel along the turns of the major helix and make contact with successive turns of the minor helix.

'A potentiometer embodying a resistance element in the form of a compound helix is disclosed in the application of Thomas B. Gibbs et al., Serial No. 573,680, filedJanuary 20, 1945, Patent No. 2,495,321, granted January 24,1950. Th potentiometer disclosed in this applicat on has'a casing of plastic materialwhich is molded around the resistance element. r

The invention is directed to the solution of a problem encountered in the manufacture of resistance elements for the potentiometers referred 3 Claims. (01. 20 1-71) V loosening of the turns 2 is attributed to the fact that when the core is wound to form the major helix it stretches somewhat and therefore is reduced in diameter. The reduction is very slight "but is sufficient to cause the difficulty above described.

The problem thus presented was solved in a novel and effective manner. The core with its winding of resistance wire is wound ona mandrel as before to form the major helix but as the winding operation proceeds the straight or unwound portion 'of the core is rotated on its own axis in the proper direction to tighten the turns of the minor helix. This operation can be performed by-hand by applying a continuous twisting torque simultaneously with the rotation of the mandrel on which the core is wound.

The invention thus make it possible to manufacture a resistance element in the f rm of a to in the preceding paragraph. The known compound helix resistance elements were wound with insulated resistance wire, which was objectionable because of the necessity of removing the insulation from the turn of the minor} helix where such turns were engaged by theslider of the potentiometer. It was proposed, therefore,

to use bare resistance wire wound on the core; of the resistance element in spaced turns. This procedure was suggested by the knowledge that the molded casing in which the turns were embedded in the completed potentiometer would hold the turns firmly in spaced relation.

It was found, however, that when the core with liable to become displaced, even with careful handling, and it was very diflicult to avoid short c rcuiting some of the turns during the preliminary operations which were necessary before the actual molding operation rould be performed. The

compound helix comprising a helically wound core with a helical winding of bare resistance wire the turns of which are tight on the core and are not liable to become displaced.

The invention has the further advantag that the resistance wire may be wound on the core in such a direction that the rotation of the core during the operation of winding the core to form the major helix not onlytightens the turns of the minor helix but also produces a major helix in which the turn are contracted tightly against each other.

' supporting and the resistance element is especially well suited for the manufacture of potentiometers in which the casing is molded around the resistance element. as described in the application previously referred to.

The invention and various features thereof will be ,described more in detail-hereinafter, reference being had to the accompanying drawing, in

which:

Fig. 1 is a plan view of a winding machine for carrying out the invention;

,Fig, 2 is across section through the core tating tube on the line 2-2, Fig. 1; v

Fig. 3 shows a resistance element according to the invention, having the form of a compound helix; and

appears before it is wound form the major helix.

The major helix is therefore self- Referring to the drawing, the various parts of the winding machine shown therein are mounted on a base II], which may be a flat steel plate adapted to rest on the top of a bench or table (not shown) to which it is secured in any suitable manner, as by means of bolts H and. 12..

The reference. character l5 indicates amandrel, which has a spiral groove thereon as shown. The mandrel l5 may be formed integrally with. the gear IS.

The mandrel and gear are rotatably supported on a fixed horizontal shaft [8, oneend of which is located in a hole in the support: ll, where it is secured by a set screw IS. The support ll extends vertically upward from the base lfl towhich it is secured by screws as shown.

The mandrel I5 is mounted on shaft I8" by means of a bearing sleeve 22. internally threaded sleeve or bushing 2| set inthe end of gear [5 which cooperates with a threaded; portion. 2!! of shaft l to enablethe latter to: function. as. a lead screw, causing the mandrel and gear. to move along shafit 18 when. they arerotated on the shaft. The pitch of the leadscrewis the'same as the pitcher". the spiral groove on the mandrel.

The means for rotating: the gear. f6 includes thetshaft 23; the gear 25 mounted on theshafiz. andthe: idler gear 26. The shait 23 rotatably mounted on the support ii and a. bearing. and is rotated by means of the hand wheel: 24: The idler. gear is rotatably mounted 'on the: headed sleeve 2'1 which is clamped against the side of a support 29 by means of a. cap screw 28.

The reference character l4 indicates the core rotating. tube which may be a steel tube having. a rather thick wall and having. a slot" 311 extend-- ing from one end of the tube to the other. The tube i4 is-rotatably mounted by means of abearing 32 secured to the base i8 and a bearing 33 which is secured to the bench or table on which base ['0 is supported. The shaft 23' rotates the tube l4 and for this purpose is provided with-a bevel gear 34 which meshes with a similar gear 35 on the tube.

'A- short cylindrical member 36 isshown in. Fig. 2 and is also indicated in Fig. l by dotted lines; This memberis slidable inside the tube I4 but cannot rotate relative to the tube; due to aprojection 3T which extends into the slotti'. lhe member 36 has a hole drilled therein of theproper size to receive the core 39 to which mem.- ber' 36 may be secured by means of the set" screw 38. V

The guide as, Fig. 1,. is mounted on base I 6 as shown, and has an opening which: is in alignmentwith the axis of tube It. This opening is of the proper size to receive a wound core 39' with a free sliding fit. The opening is preferably tapered at both ends; I

- Details of the construction ofi the core. 39' and. its winding 43 of bare. resistance wire are'showni in Fig; 4. The core may be a section of No. 14 copper wire 4!, having an insulating coating 4'2 of suitable insulating material suchas Formvar or Formex. The resistance wiremay be about 2 mils in diameter; for. example, and is wound. onthe core .353. in. spacedturns while. the. core. is: straight. Any suitable andlknown type of wind ing machine may be used" for this purpose}. The. endsof the. winding are. secured by solderinga few turns of wire together as. indicated at M, which prevents. the coils from.- unwinding. The. core '39" should" be" somewhat. longer. than the.

There is also an. I

4 winding so that there will be an unwound section several inches in length at each end.

Other sizes of core wire and resistance wire may of course be used, the sizes specified above being given merely by way of example.

In order to" prepare themachine for operation,.the; operator turns the'hand wheel 24 in the proper direction to traverse the gear I6 and mandrel l5 along shaft 18 until these parts are in the. position in which they are shown in the drawing, with the end of the gear approximately in engagement with the support 17.

There is a. radial. hole drilled near the end of the mandrel and this hole now appears on top and in alignment with the opening in the guideAfiL.

The operator now takes a wound core 39 and passes the unwound section at one end through thefguide 49, from the left, for a short distance. The cylindrical member 36 is now placed on the end of thecore and rthe set screw 38. is: turned int-ightly, thus securing member 36 to the' core; The" member: 36 is. then inserted: in. the end of tube It and thecore and the said member are pushed into the tubeuntil only about 6 inches or so of: the core: is left projecting to the. leiti beyond the mandrel l5; With: a pair of. pliers the operator now. bends theend of the coredownward to form a hook as shown in Fig. 4 andtheni pushes the core-enough farther. into'thei tube to bring the hook to" a position where it. can; enter: the hole inthe mandrel, being pressed into the. holegif-necessary, The apparatus is now in. con dit-ion for the winding operation to begin.

The operator-turns the hand wheelin. the die rection shown by the. arrow, or in a counter.- clockwise direction as seen from thefront of. the. machine, and the gear. I 6 and mandrel ti are; rotated in. the same direction; The rotationof the mandrel causes the core: 39 to be drawn out of the tube HI and. wound. up on. the mandrel. in. the form of a helix, the turns of which-are spaced apart and laid intoithe helical: grooveon': the mandrel by the action. of the lead screwrZB, which advances themandrel along the. shafmh as the winding progresses;

:The. rotation of thehan'd wheel 24- also rotates the tube .M, which; rotatesthemember 36 and. the core 39 to whichit' is secured. in this connection it will .be recalled. that the member 36 is slidable along the tube-but is. constrained!- to rotate withthe tube by the projection 3?- located in. the slot 31. -The-rotation of the core 39" has: no.-permanent effect along the straight section oft-hecore, which merely serves to: communicate the torque tothe point OIL-the: core where-it is being applied to the mandrel.- At this point a: simultaneous bending andtwisting of. the core. takes: place in. excess of the elastic limit of. the material and the core is given apermanent. set. That is, as the winding proceeds: the. straight section. of the core is permanently rotated; or; twisted relative to the section already wound on I the. mandrel.

Where ha s: en n a t. son. ta

mandrel and where, as previously explained, the

straight section of the core is permanently twisted relative to the section which has been wound on the mandrel. This action is continuous and is actually a winding action for each rota- 5 tion of the core adds one turn to the resistance wire winding 43.

In the machine shown in, the drawing the gear ratio between the mandrel I5 and tube I4 is 1 to 1 and accordingly one turn is added to the minor helix for each turn of the major helix. This gear ratio has been found to be satisfactory for winding a core wire of the size mentioned into a major helix having an outside diameter of about one and one half inches. For other sizes of core and helix the gear ratio might have to be changed. In any case the tube l4 should be rotated at such a speed relative to the speed of the mandrel that loosening of theturns of the resistance wire on the core is prevented. The speed of the tube may be high enough to actually tighten the turns somewhat but should not be so high as to unduly strain the resistance wire.

The operator continues the operation until the entire wound portion of the core 39 has been wound up on the mandrel. At this time the cylindrical member 36 will have emerged from the tube [4 and can be removed from the end of the core by loosening the set screw 38. The hand wheel is then rotated a little more, suflicient to draw the end of the core through the guide 50, and the end is pressed down against the mandrel.

This completes the winding operation. The turns of the helix into which the core has been formed tend to contract and hug the mandrel tightly. In order to remove the core from the Lin * mandrel the operator lifts the hook at the end of the first turn out of the hole in the mandrel and then while holding the core with one hand rotates the hand wheel 24 in a direction opposite to that in which it was rotated during the winding operation. This unscrews the mandrel from the core and at the same time brings it into the proper position for winding another core.

All that now remains to be done is to out off the unwound end sections of the core close to the ends of the winding 43 and the resistance element is completed. Fig. 3 shows the complete resistance, element.

As already mentioned, a resistance element made in accordance with the invention is especially well adapted for the manufacture of potentiometers by a process in which the casing is molded around the resistance element, as describedin the Gibbs et al. patent previously referred to. In this process a mold is used which has a helically grooved core for supporting the resistance element during the molding operation. Due to the fact that the turns or coils of the major helix of the resistance element are tensioned against each other the resistance element may be simply screwed on to the core where it is retained securely without any support other than that afforded by the contraction of the coils on the core of the mold. Of perhaps greater importance is the fact that the turns or coils of the resistance wire winding 43 are so tight on the core 39 that the operation of screwing the resistance element on the core of the mold, or other handling to which the resistance element may be subjected, involves no danger of displacing the coils and enables these operations to be carried out without any special precautions in this regard.

While a particular use for the resistance element has been pointed out, it will be clear that it can be used to advantage in various other types of rheostats and potentiometers.

The invention having been described, that which is believed to be new and for which the protection of Letters Patent is desired will be ponited out in the appended claims.

I claim.

1. A resistance element in the form of a compound helix, comprising an insulated metallic core, a helical winding of resistance wire on said core, said winding constituting a minor helix, said core having the form of a helical winding constituting a major helix, said core comprising successive unit sections each of which is permanently angularly displaced from the preceding section by rotation of the core during formation of said major helix, the direction of rotationbeing such that the turns of the major helix contract against each other, and the direction of winding of said minor helix being such that the said rotation of the core has tightened the turns of the minor helix on the core.

2. A resistance element in the form of a compound helix, comprising a core having a winding of resistance wire thereon, said winding constituting a minor helix, said core having the form of a helical winding constituting a major helix, said 1 minor helix being under tension produced in part at least by the said deformation of the core.

3. A resistance element in the form of a compound helix, comprising a core having a winding of resistance wire thereon, said winding constituting a minor helix, said core having the form of a helical. winding constituting a major helix, said core being made of relatively soft inelastic metal and having a permanent torsional set produced [by a twisting operation which strained the metal beyond its elastic limit during the winding of said major helix, and the minor helix having a tension produced by the twisting operation and retained by said permanent set which causes the turns of the minor helix to grip the core.

BERNARD A. WARMEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Marlow Nov. 23, 1943 

